1. Priority
Priority is claimed to International Patent Application No. PCT/AU03/00561, filed on May 9, 2003 and published as International Publication No. WO 03/096072 A1 on Nov. 20, 2003, which claims priority to Australian Application No. 2003900266, filed on Jan. 20, 2003, and Australian Provisional Application No. PS2255, filed on May 10, 2002. The disclosures of these priority documents are incorporated herein by reference.
2. Field of the Invention
The present invention relates generally to deploying seafloor equipment, for example seismic recorders for use in marine seismic surveying. While the invention will be described hereinafter with reference to this application, it will be appreciated that the invention is not limited to this particular field of use.
3. Description of the Prior Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
For various applications it may be necessary to deploy equipment to the seafloor. For example, seafloor recorders are often used for earthquake monitoring or marine seismic operations. These devices are typically referred to as “Ocean Bottom Seismometers” and various descriptions can be found in U.S. Pat. No. 4,692,906 to Neeley (1987), U.S. Pat. No. 5,189,642 to Donoho et al. (1993) and U.S. Pat. No. 5,253,223 to Svenning et al. (1993). Seafloor recorders typically consist of a pressure resistant waterproof container housing: a clock, digital data recording electronics, a battery, three geophones to sense the seafloor movement in all directions and a hydrophone to sense acoustic pressure. They can also be equipped with other means such as a chassis for coupling to the ground, a recovery module usually based on a weight release mechanism to ascend back to the surface, and secondary sensors such as a magnetic heading sensor, a tilt sensor and depth sensor.
Various methods of deploying seafloor recorders have been proposed for applications such as oil exploration geophysics which require very high quality geologic images to be obtained from seismic signals acquired at the seabed. Nevertheless, the imaging requires a reasonable control of the positioning of the sensor during deployment, which is a significant issue in deep water, or in presence of strong currents. For instance, U.S. Pat. No. 5,253,223 to Svenning et al. (1993) disclosed a submarine vessel to deploy the recorders. U.S. Pat. No. 6,244,375 B1 to Norris et al. (2001) disclosed a method using recorders travelling autonomously along predefined paths, such as tubing, laid at the ocean bottom. Those methods require a very significant and expensive infrastructure to be put in place.
U.S. Pat. No. 6,024,344 to Buckley et al. (2000) disclosed a method for recording seismic data in deep water where a plurality of seismic data recorders are attached to a wire stored on a seismic vessel. A free end of the wire is deployed into the water, and the recorders are attached at selected positions along the wire. The wire and recorders are lowered into the water as the vessel moves to control the recorder deployment. The wire controls recorder location and establishes the recorder spacing interval. One significant drawback of this method is that for effective deployment in presence of currents, the density and mass per unit length of the cable and recorders has to be high compared to hydrodynamic drag, which results in a very significant overall weight to be carried by the vessel.